No, it's not that.<quoted text>
To conform with the extruded cylindrical pipe beneath.
Plus, it means that you don't have to turn the cover to fit the opening as it is universally symmetrical.
See, if a manhole cover is a square of side a then the length of its diagonal is a x (2)^1/2 or a times square root 2 = a x 1.41 = 1.41 a.
This diagonal length (1.41 a) of a square is greater by a factor 0.41 when compared to its side which is a or by 41 % and so the square cover can easily slide into the manhole along the diagonal and will have to be replaced often and besides it is also a waste of metal to construct a shape like a square that has a larger surface area.
On the other hand, if a manhole cover is circular, its semi-perimeter or half its circumference is (2 x pi x r)/2 = pi x r = 3.14 x r, whereas the diameter of a circle is 2r.
So, we clearly see that 3.14 r (half the circumference or any half of the round cover) is greater in measure than the diameter of the circle (2r) by a factor: 3.14r - 2r = 1.14r.
So, the circular cover of the manhole will not slide into the manhole and besides the circular shape has a smaller surface area and thus would use less metal for its fabrication.
This is my logic.